1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography (MRT) as employed in medicine for examining patients. The present invention is thereby directed to an apparatus for processing and presenting a magnetic resonance tomography measured image as well as to an imaging MR method.
2. Description of the Prior Art
Magnetic resonance tomography is a tomographic method for medical diagnostics that is mainly distinguished by a high contrast resolution capability. Due to the excellent presentation of soft tissue, magnetic resonance tomography has developed into a method that is often superior to x-ray computed tomography. Magnetic resonance tomography is currently based on the application of spin echo sequences and gradient echo sequences that enable an excellent image quality given measuring times on the order of magnitude of minutes.
It is particularly the hydrogen nuclei, that are abundantly present in biological tissue, that enable the production of medically meaningful images. However, heavier magnetic nuclei such as, for example, 13C, 19F, 23Na, 31P can also be detected in biological tissue and imaged analogous to the hydrogen nucleus despite their lower concentration. The resonant frequencies of the most important nuclei occurring in biological tissue and their relative detection sensitivity given the same measuring frequency and taking their natural occurrence into consideration are shown by FIG. 3.
In experiments wherein the investigated nuclei are incorporated into different molecules, however, slightly different resonant frequencies are observed given the same magnetic field. This is due to the electrons in the molecule that cause a phenomenon referred to as the “chemical shift”. This chemical shift is the property that the resonant frequency is shifted slightly proportionately to the field strength dependent on the type of chemical bond in which the nucleus is situated.
As an example, FIG. 4 shows the phosphorous spectrum of the human thigh muscle at 2T. The metabolites adenosine triphosphate (ATP), creatine phosphate (PCr), in organic phosphate (Pi) and phosphorous diester (PDE) can be distinguished from one another on the basis of their chemical shifts.
Particularly when registering the resonant frequency of hydrogen, artifacts occur at the boundary layers between fat and water in the presentation of the tissue of patients, these artifacts arising from the influence of the chemical shift. Due to their high concentration in the human body, it is mainly hydrogen nuclei of free water and of fat that contribute to the image. Their relative resonant frequency different Δf amounts to approximately 3 ppm (parts per million). The Δf leads to a relative shift of the images of the two nuclei in the direction of the gradient that is active during the data registration (“read gradient” or “frequency coding gradient”). The extent of the shift is dependent on the bandwidth employed per pixel, which is in turn dependent, among other things, on the field of view and on the matrix size.
In order to facilitate orientation within the anatomy for the user, there is therefore the demand that the signal of one spin type be suppressed either entirely or up to a certain extent.
In general, the fat signal is suppressed because the critical diagnostic information can be obtained from the water signal. Fading in the fat signal (or the incomplete suppression thereof) serves the purpose of anatomical orientation (for example, in orthopedics).
In the registration of the nuclear resonance of hydrogen nuclei, the display of the water image with a permanently set degree of fat suppression is standard. This standard method utilizes the frequency shift between water and fat in order to emit a selective, narrowband RF pulse that only acquires one of the two spin type—preferably fat—and rotates by an angle α≦90° in the transverse plane. Due to the application of a suitable gradient pulse (spoiler gradient), the transverse magnetization is completely dephased and only the longitudinal spin portion is still coherent. At α=90°, the entire fat part is suppressed since a longitudinal part is no longer present after the application of the RF pulse.
According to the above method, however, the degree of suppression accompanying the presented image was only able to be permanently set by the user before the exposure; variation after the end of the measurement is not possible.